1. Field of the Invention
This invention relates generally to semiconductor manufacturing and more particularly to real time metrology for process and wafer state monitoring in semiconductor manufacturing operations.
2. Description of the Related Art
During semiconductor fabrication, there exists multiple steps where an underlying substrate is subjected to the formation, modification and removal of various layers, blanket or patterned. The small feature sizes, tight surface planarity requirements, combined with the constant quest to increase throughput, makes it highly desirable to stop or change the process parameters when the targeted properties (thickness, resistance, planarity, transparency, chemical composition etc.) of the processed film has been achieved, i.e., when an endpoint has been obtained for the current process step. Of course, some semiconductor fabrication steps transition to a subsequent fabrication step after the current process step has accomplished the task of obtaining the defined wafer characteristics.
Real time metrology for the control of wafer characteristics is now a necessity so that an endpoint or transition point for a particular processing operation may be determined. The real time in-situ monitoring of parameters associated with a semiconductor operation provides valuable information as to an endpoint or a transition point of a processing operation. Typically the properties of an object, either the wafer itself or another object, which is strongly linked to the wafer in the process, undergo monotonic change prior to and after the transition, experiencing an abrupt property variation during the transition itself. This leads to a step-like property for monitoring signal variation in cases when the monitored system is small enough and when the transition occurs simultaneously for the watch point of the inspection space. For larger systems, such as semiconductor wafers, there is a time distribution, which transfers a step-like transition point into a slope change, associated with data points corresponding to the process parameter changes, thereby creating an indicator of the endpoint or transition point. However, slope change detection requires usage of derivatives, which are associated with reduced signal-to-noise ratio complicating this approach.
FIG. 1 is a graph of a thickness of a semiconductor wafer being monitored over time during a processing operation, such as a planarization processing operation. Line 100 represents values associated with an infrared (IR) based sensor for determining an endpoint/transition point during a processing operation. Lines 102 represent values associated with a plurality of eddy current sensors (ECS) for capturing thickness of a semiconductor wafer over time during the processing. As can be seen, region 104 represents the time where the endpoint/transition point occurs. In region 104, the general slope associated with lines 102 and line 100 transition. However, the signal being monitored for either the IR monitoring or eddy current monitoring is superimposed with significant and variable background noise that changes from run to run. Accordingly, when measuring the derivative based slope to determine the endpoint, the signal to noise level affects the stability and reliability of the endpoint determination. Thus, in-line determination of a transition point predicated upon a slope based analysis does not provide the robust data necessary for semiconductor operations.
As a result, there is a need to solve the problems of the prior art to provide a method and apparatus for providing stable and reliable transition point determination for processes where the transition point occurs through a slope based transition of a monitored process parameter.